Prometheus: Disable exemplar queries for alerting (#41607)

* Prometheus: Dont include empty exempalr frame in results

* Prometheus: Never run exemplar queries for alerting

* Remove exemplar field from alerting and set exemplar to false

* Add tests for frontend

* Add test for backend

pkg/tsdb/prometheus/time_series_query.go

@@ -195,6 +195,12 @@ func (s *Service) parseTimeSeriesQuery(queryContext *backend.QueryDataRequest, d
 			rangeQuery = true
 		}
 
+		// We never want to run exemplar query for alerting
+		exemplarQuery := model.ExemplarQuery
+		if queryContext.Headers["FromAlert"] == "true" {
+			exemplarQuery = false
+		}
+
 		qs = append(qs, &PrometheusQuery{
 			Expr:          expr,
 			Step:          interval,
@@ -204,7 +210,7 @@ func (s *Service) parseTimeSeriesQuery(queryContext *backend.QueryDataRequest, d
 			RefId:         query.RefID,
 			InstantQuery:  model.InstantQuery,
 			RangeQuery:    rangeQuery,
-			ExemplarQuery: model.ExemplarQuery,
+			ExemplarQuery: exemplarQuery,
 			UtcOffsetSec:  model.UtcOffsetSec,
 		})
 	}

pkg/tsdb/prometheus/time_series_query_test.go

@@ -59,6 +59,37 @@ func TestPrometheus_timeSeriesQuery_parseTimeSeriesQuery(t *testing.T) {
 		intervalCalculator: intervalv2.NewCalculator(),
 	}
 
+	t.Run("parsing query from unified alerting", func(t *testing.T) {
+		timeRange := backend.TimeRange{
+			From: now,
+			To:   now.Add(12 * time.Hour),
+		}
+
+		queryJson := `{
+			"expr": "go_goroutines",
+			"refId": "A",
+			"exemplar": true
+		}`
+
+		query := &backend.QueryDataRequest{
+			Queries: []backend.DataQuery{
+				{
+					JSON:      []byte(queryJson),
+					TimeRange: timeRange,
+					RefID:     "A",
+				},
+			},
+			Headers: map[string]string{
+				"FromAlert": "true",
+			},
+		}
+
+		dsInfo := &DatasourceInfo{}
+		models, err := service.parseTimeSeriesQuery(query, dsInfo)
+		require.NoError(t, err)
+		require.Equal(t, false, models[0].ExemplarQuery)
+	})
+
 	t.Run("parsing query model with step", func(t *testing.T) {
 		timeRange := backend.TimeRange{
 			From: now,